In the first year of this project, we have examined the normal structure, particularly the ultrastructure of the arachnoid villus, in the cat under conditions of normal intracranial pressure. We have shown that arachnoid villi in the cat exist as both simple arachnoid villi which protrude into the dural wall of the superior sagittal sinus, as well as more complex arachnoid granulations which protrude through the wall and extend into the lumen of the sinus. In both cases, the villus is completely covered by the endothelium lining the sinus, this endothelium serving as a barrier to CSF transport across the villus into the venous blood. We have shown the arachnoid villus endothelium to be a single layer of cells containing numerous micropinocytotic vesicles which have also been shown to transport horseradish peroxidase, when injected into the CSF, across the villus endothelium into the sinus blood. Furthermore, we have shown that the endothelial cells are joined in most part by tight junctions which effectively prohibit the transfer of HRP across the intraendothelial spaces. Using short-duration low volume intrathecal infusions of artificial CSF, we have established a series of steady-state CSF pressures in different groups of animals from which we have calculated outflow resistance under various anesthetic regimens. We have shown that at resting pressures of 40-80 mm/H2O, as well as elevated pressures of 200 and 500 mm of CSF pressure, outflow resistance is highly nonlinear, suggesting that at higher pressures a proportionately greater amount of CSF is transferred out of the CSF system than at lower CSF pressures.